Memory storage density is a measure of the quantity of information (i.e., bits) that can be stored on a given length of track, area of surface, or in a given volume of a computer storage medium. Higher density is more desirable because it allows greater volumes of data to be stored in the same physical space. Density therefore has a direct relationship to storage capacity of a given medium. Density also generally has a fairly direct effect on the performance within a particular medium, as well as price. Hard drives store data in the magnetic polarization of small patches of the surface coating on a (normally) metal disk. The maximum areal density is constrained by the size of the magnetic particles in the surface, as well as the size of the “head” used to read and write the data.
To increase areal density, magnetic recording systems have begun using “shingled” writing schemes, in which newly written tracks partially overwrite previously written tracks. This has the effect of narrowing the previously written tracks, with resulting widths less than the width of the write head. As a result, track density and therefore areal density is increased.
However, as areal density increases, read-back of the magnetic polarization areas becomes increasingly difficult. In particular, the strength of the signal generated by the magnetic polarization of small patches of the surface of the metal disk decreases while interference from adjacent tracks increases.
Therefore, as magnetic writing technique make possible further increases in areal density, techniques must be developed to allow the smaller sized bits to be successfully read back from the storage medium.